Transparent polyester film having a high oxygen barrier and process for its production

ABSTRACT

The invention relates to a transparent, biaxially oriented polyester film with a base layer B, at least 80% by weight of which is composed of a thermoplastic polyester, and with at least one outer layer A. The outer layer A is composed of a copolymer or a mixture of polymers/copolymers which contains ethylene 2,6-naphthalate units in a range of from 90 to 98% by weight and up to 10% by weight of ethylene terephthalate units, and/or units derived from cycloaliphatic or aromatic diols and/or dicarboxylic acids. Its thickness is more than 0.7 μm and makes up less than 25% by weight of the total film and the T g 2 value of the polyester film is above the T g 2 value of the base layer, but below the T g 2 value of the polyester for the outer layer. The transparent film has low permeability to atmospheric oxygen and exhibits very good adhesion between the individual layers. It is particularly suitable for packaging purposes, specifically for packaging foods or other consumable items.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a transparent, biaxially orientedpolyester film with a base layer B, at least 80% by weight of which iscomposed of a thermoplastic polyester, and with at least one outer layerA. The invention further relates to the use of the film and to a processfor its production.

[0002] Prior Art

[0003] EP-A-0 878 297 describes a transparent, biaxially orientedpolyester film with a base layer B, at least 80% by weight of which iscomposed of a thermoplastic polyester, and with at least one outer layerA which is composed of a mixture of polymers which contains at least 40%by weight of ethylene 2,6-naphthalate units and up to 40% by weight ofethylene terephthalate units and/or up to 60% by weight of units derivedfrom cycloaliphatic or aromatic diols and/or dicarboxylic acids.

[0004] If the outer layer A of the film of EP-A-0 878 297 contains largeamounts of ethylene 2,6-naphthalate units, the film has a tendency fordelamination between the outer layer A and the base layer B. If, on theother hand, the outer layer A contains low concentrations of ethylene2,6-naphthalate units, the thickness of this layer has to be raised inorder to achieve the low oxygen permeation of not more than 80cm³/(m²•bar•d).

[0005] In a film in Example 8 of EP-A-0 878 297 the outer layer A usespure polyethylene 2,6-naphthalate (corresponding to 100% by weight ofethylene 2,6-naphthalate units). In this case there is no significantadhesion between the outer layer A and the base layer B. The film isunsuitable for industrial use (e.g. as a composite film), since the bondreleases even when subjected to a low level of mechanical stress, due tothe low adhesion between the outer layer A and the base layer B of thepolyester film.

[0006] In a film in Example 11 of EP-A-0 878 297, the outer layer Acontains 60% by weight of ethylene 2,6-naphthalate units. In order toachieve the low oxygen permeation demanded, below 80 cm³/(m²•bar•d), thethickness of the outer layer A has to be raised to 3 μm, and this iseconomically disadvantageous (high capital expenditure and high materialcosts).

[0007] U.S. Pat. No. 5,795,528 describes a coextruded film laminatewhich has alternating layers of PEN and PET. Like the film of EP-A-0 878297, this film has a tendency toward delamination between the individuallayers of PEN and PET. There is no significant adhesion between theselayers. A laminate of this type is therefore again unsuitable forindustrial use.

[0008] It was an object of the present invention, therefore, to providea transparent, biaxially oriented polyester film which overcomes thedisadvantage of the prior art films and in particular has improvedadhesion between the individual layers. The film should be simple andcost-effective to produce, have good barrier properties, and pose noproblems of disposal.

SUMMARY OF THE INVENTION

[0009] This object is achieved by means of a transparent, biaxiallyoriented polyester film with a base layer B, at least 80% by weight ofwhich is composed of a thermoplastic polyester, and with at least oneouter layer A, the characterizing features of which are regarded asbeing that

[0010] the outer layer A is composed of a copolymer or of a mixture ofhomopolymers and copolymers, which contains ethylene 2,6-naphthalateunits in an amount in the range of from 90 to 98% by weight and up to10% by weight of ethylene terephthalate units, and/or units derived fromcycloaliphatic or aromatic diols and/or dicarboxylic acids;

[0011] the thickness of the outer layer A is more than 0.7 μm andcorrespondingly makes up less than 25% by weight of the total film; and

[0012] the T_(g)2 value of the polyester film is above the T_(g)2 valueof the base layer but below the T_(g)2 value of the outer layer.

[0013] The film of the invention has low oxygen permeation, below 85cm³/(m² bar •d), and minimum adhesion between the individual layers Aand B of greater than or equal to 0.5 N/25 mm.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The structure of the film of the invention has at least twolayers, and is then composed of an outer layer A and of a base layer B.

[0015] For the purpose of this invention a polymer consisting of onetype of dicarboxylic acid and one type of diol; e.g. polyethyleneterephthalate, is called a “homopolymer”. If it contains a second, thirdor more dicarboxylic acid and/or diol it is called a “copolymer”.

[0016] Preference is given to a polyester film in which the copolymer orthe mixture of homopolymers and copolymers of the outer layer A containsethylene 2,6-naphthalate units in a range of from 91 to 97% by weightand up to 9% by weight of ethylene terephthalate units and/or unitsderived from cycloaliphatic or aromatic diols and/or dicarboxylic acids.Among these, particular preference is in turn given to a polyester filmin which the copolymer or the mixture of homopolymers and copolymers ofthe outer layer A contains ethylene 2,6-naphthalate units in a range offrom 92 to 96% by weight and up to 8% by weight of ethyleneterephthalate units.

[0017] Preference is also given to a polyester film whose outer layer Ahas a thickness of more than 0.8 μm and correspondingly makes up lessthan 22% by weight of the total film, and particular preference is givento a polyester whose outer layer A has a thickness of more than 0.9 μmand correspondingly makes up less than 20% by weight of the total film.

[0018] Examples of suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol), or branchedaliphatic glycols having up to 6 carbon atoms, and cycloaliphatic diolshaving one or more rings and if desired containing heteroatoms. Amongthe cycloaliphatic diols, mention may be made of cyclohexanediols (inparticular 1,4-cyclohexanediol). Examples of other suitable aromaticdiols are those of the formula HO—C₆H₄—X—C₆H₄—OH where X is —CH₂—,—C(CH₃)₂—, —C(CF₃)₂—, —O—, —S—or —SO₂—. Besides these, bisphenols of theformula HO—C₆H₄—C₆H₄—OH are also very suitable.

[0019] Preferred aromatic dicarboxylic acids are benzenedicarboxylicacids, naphthalenedicarboxylic acids (for example naphthalene-1,4- or-1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), diphenylacetylene-x,x′-dicarboxylicacids (in particular diphenylacetylene-4,4′-dicarboxylic acid) orstilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylicacids, mention may be made of cyclohexanedicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids, the C₃-C₁₉-alkanedioic acids are particularlysuitable, where the alkane moiety may be straight-chain or branched.

[0020] The base layer of the film is preferably composed of at least 90%by weight of the thermoplastic polyester. Polyesters suitable for thisare those made from ethylene glycol and terephthalic acid (=polyethyleneterephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (=polyethylene 2,6-naphthalate, PEN),from 1,4-bishydroxymethylcyclohexane and terephthalic acid(=poly-1,4-cyclohexanedimethylene terephthalate, PCDT), and also fromethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters whichare composed of at least 90 mol %, preferably at least 95 mol %, ofethylene glycol units and terephthalic acid units or of ethylene glycolunits and naphthalene-2,6-dicarboxylic acid units. The remaining monomerunits are derived from other diols and/or dicarboxylic acids. Examplesof suitable diol comonomers are diethylene glycol, triethylene glycol,aliphatic glycols of the formula HO—(CH₂)n—OH, where n is an integerfrom 3 to 6, branched aliphatic glycols having up to 6 carbon atoms,aromatic diols of the formula HO—C₆H₄—X—C₆H₄—OH where X is —CH₂—,—C(CH₃)₂—, —C(CF₃)₂—, —O—, —S—or —SO₂—, or bisphenols of the formulaHO—C₆H₄—C₆H₄—OH are employed.

[0021] The dicarboxylic acid comonomer units are preferably derived frombenzenedicarboxylic acids, naphthalenedicarboxylic acids,biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), cyclohexane-dicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid), stilbene-x,x′-dicarboxylicacid or C₁-C₁₆-alkanedicarboxylic acids, where the alkane moiety may bestraight-chain or branched.

[0022] The polyesters may be prepared by the transesterificationprocess. The starting materials for this are dicarboxylic esters anddiols, which are reacted using the customary transesterificationcatalysts, such as salts of zinc, of calcium, of lithium and ofmanganese. The intermediates are then polycondensed in the presence ofwidely used polycondensation catalysts, such as antimony trioxide ortitanium salts. The preparation may be carried out just as successfullyby the direct esterification process in the presence of polycondensationcatalysts, starting directly from the dicarboxylic acids and the diols.

[0023] The copolymers for the outer layer A may be prepared in threedifferent ways:

[0024] a) In copolycondensation, terephthalic acid andnaphthalene-2,6-dicarboxylic acid are placed in a reactor together withethylene glycol, and polycon-densed to give a polyester, using thecustomary catalysts and stabilizers. The terephthalate and naphthalateunits are then randomly distributed in the polyester.

[0025] b) Polyethylene terephthalate (PET) and polyethylene2,6-naphthalate (PEN), in the desired ratio, are melted together andmixed, either in a reactor or preferably in a melt kneader (twin-screwkneader) or in an extruder. Immediately after the melting,transesterification reactions between the polyesters begin. Initially,block copolymers are obtained, but as reaction time increases—dependingon the temperature and mixing action of the agitator—the blocks becomesmaller, and long reaction times give a random copolymer. However, it isnot necessary and also not always advantageous to wait until a randomdistribution has been achieved, since the desired properties are alsoobtained with a block copolymer. The resultant copolymer is thenextruded from a die and granulated.

[0026] c) Polyethylene terephthalate (PET) and polyethylene2,6-naphthalate (PEN) are mixed as granules in the desired ratio, andthe mixture is fed to the extruder for the outer layer A. Here, thetransesterification to give the copolymer takes place directly duringthe production of the film. This process has the advantage of being verycost-effective, and generally gives block copolymers, the block lengthbeing dependent on the extrusion temperature, the mixing action of theextruder and the residence time in the melt.

[0027] In a preferred embodiment of the invention, from 0.1 to 20% byweight of the polymers of the base layer B are identical with those ofthe outer layer A. These are either directly admixed with the base layerB during extrusion or are in any case present in the film due toaddition of regrind. The proportion of these copolymers in the baselayer is selected in such a way that the base layer has crystallinecharacter.

[0028] In another advantageous embodiment, the film encompasses, on theside facing away from the outer layer A, another outer layer C ofpolyethylene terephthalate, and this layer comprises pigments.

[0029] The film of the invention has a high oxygen barrier and thedesired good adhesion between its individual layers.

[0030] If the polymers/copolymers used for the outer layer A containless than 90% of ethylene 2,6-naphthalate units and more than 10% byweight of ethylene terephthalate units along with a thickness of theouter layer A of less than 0.7 μm, although the film then has lesspermeability to oxygen than a standard polyester film (composed of 100%by weight of polyethylene terephthalate), its permeability is still toohigh for the purposes of the present invention.

[0031] If the polymers/copolymers used for the outer layer A containmore than 98% by weight of ethylene 2,6-naphthalate units (for examplepure polyethylene 2,6-naphthalate), the adhesion between the outer layerA and the base layer B becomes inadequate. When subjected to mechanicalstress the film tends toward delamination, which is undesirable andmakes the film unusable.

[0032] A difference from the prior art is that in the film of theinvention moreover the glass transition temperature T_(g) of thecopolymer or of the mixture of homopolymers and copolymers of the outerlayer A is higher than the glass transition temperature T_(g) of thepolymers for the base layer B. The glass transition temperature T_(g) ofthe copolymers used for the outer layer A is preferably in the rangefrom 90 to 120° C. In the DSC (differential scanning calorimetry)determination of the glass transition temperatures T_(g), thetransitions of the individual layers cannot be differentiated in acomposite film.

[0033] Glass transitions which are determined on biaxially oriented,heat-set films in the first heating procedure (termed T_(g)1 below) are,due to crystallinity and also to molecular stresses in the amorphousfraction of the specimens, relatively small in size, distributed over awide temperature range, and shifted to higher temperatures. Because oforientation effects in particular, they are not suitable forcharacterizing a polymer as such. The resolution of DSC analyzers isoften insufficient to detect the glass transitions in the first heatingprocedure (T_(g)1) of the individual layers of the film of the inventionat all, the transitions being “blurred” and small, due to orientationand crystallinity.

[0034] If, however, the specimens are melted and then rapidly cooledagain to below their glass transition temperature T_(g) (quenched), theorientation effects are eliminated. On renewed heating, glasstransitions (designated T_(g)2 here) are then measured which have agreater intensity and whose temperature is characteristic of therespective polymers. However, even then it is not possible todifferentiate the glass transitions of the polyesters of the individuallayers, since the layers mix on melting and the polyesters presenttherein enter into transesterification reactions with one another. It isfully sufficient, however, to compare the temperature T_(g)2 of theentire coextruded films with the temperature ^(T) _(g)2 of the polymerused for the base layer B. In known films the T_(g)2 value of thepolyester for the base layer is higher than that of the coextruded film,whereas the T_(g)2 value of the polyester for the outer layer is lowerthan that of the polyester for the base layer and also lower than thatof the coextruded film. Exactly the opposite of this applies for thefilm of the invention. Here, the T_(g)2 value of the coextruded film ishigher than that of the polyester for the base layer B but below theT_(g)2 value of the polyester for the outer layer A.

[0035] The base layer B and the outer layer(s) may also comprisecustomary additives, such as stabilizers and antiblocking agents. Theyare expediently added to the polymer or to the polymer mixture evenbefore melting takes place. Examples of stabilizers are phosphoruscompounds, such as phosphoric acid and phosphoric esters. Typicalantiblocking agents (also termed pigments in this context) are inorganicand/or organic particles, for example calcium carbonate, amorphoussilica, talc, magnesium carbonate, barium carbonate, calcium sulfate,barium sulfate, lithium phosphate, calcium phosphate, magnesiumphosphate, aluminum oxide, LiF, the calcium, barium, zinc and manganesesalts of the dicarboxylic acids used, carbon black, titanium dioxide,kaolin, crosslinked polystyrene particles and crosslinked acrylateparticles.

[0036] The additives selected may also be mixtures of two or moredifferent antiblocking agents or mixtures of antiblocking agents of thesame makeup but of different particle size. The particles may be addedto the individual layers in the customary concentrations, e.g. asglycolic dispersion during polycondensation or via masterbatches duringextrusion. Pigment concentrations of from 0.0001 to 5% by weight haveproven advantageous. A detailed description of the antiblocking agentsis found, for example, in EP-A-0 602 964.

[0037] The film may be coated and/or corona- or flame-pretreated toestablish other desired properties. Typical coatings are layers whichpromote adhesion, are antistatic, improve slip, or have release action.These additional layers may be applied to the film by in-line coatingusing aqueous dispersions, before transverse orientation. The film isalso particularly suitable for metallizing or coating with ceramicsubstances (SiO_(x), Al₂O₃). Particularly good oxygen barrier values areachieved if the outer layer A is metallized or ceramic-coated.

[0038] The polyester film of the invention may preferably also comprisea second outer layer C. The structure, thickness, and chemical makeup ofthe second outer layer C may be selected without reference to the outerlayer A already present, and the second outer layer C may also comprisethe abovementioned polymers or polymer mixtures, but these do notnecessarily have to be identical with the polymers for the first outerlayer A. The second outer layer C may also comprise other commonly usedouter layer polymers.

[0039] Between the base layer B and the outer layer A, there may also bean intermediate layer Z. The thickness of the intermediate layer isgenerally above 0.1 μm and is preferably in the range from 0.2 to 20 μm,particularly from 0.3 to 10 μm.

[0040] The thickness of the outer layer C is generally above 0.1 μm,preferably in the range from 0.2 to 5 μm, and particularly from 0.2 to 4μm, and the thicknesses of the outer layers may be identical ordifferent.

[0041] The total thickness of the polyester film of the invention mayvary within wide limts and depends on the application envisaged. It isfrom 6 to 100 μm, preferably from 8 to 50 μm, particularly preferablyfrom 10 to 30 μm, the proportion of the total thickness made up by thebase layer B preferably being from about 40 to 95%.

[0042] The present invention also provides a process for producing thisfilm. It encompasses

[0043] a) producing a film having two or more layers from a base layer Band outer layer(s) A and, where appropriate, C, by coextrusion;

[0044] b) biaxially stretching the film, and

[0045] c) heat-setting the stretched film.

[0046] To produce the outer layer A, it is expedient to feed granules ofpolyethylene terephthalate and polyethylene 2,6-naphthalate directly tothe extruder in the desired mixing ratio. At about 300° C., the twomaterials can be melted and can be extruded. Under these conditions,transesterification reactions can occur in the extruder and during thesecopolymers are formed from the respective homopolymers.

[0047] The polymers for the base layer B are expediently fed in viaanother extruder. Any foreign bodies or contamination which may bepresent can be filtered off from the polymer melt before extrusion. Themelts are then extruded through a coextrusion die to give flat meltfilms and are layered one upon the other. The composite film is thendrawn off and solidified with the aid of a chill roll and other rolls ifdesired.

[0048] The biaxial stretching is generally carried out sequentially,stretching first longitudinally (i.e. in the machine direction) and thentransversely (i.e. perpendicularly to the machine direction). This leadsto orientation of the molecular chains within the polyester. Thelongitudinal stretching can be carried out with the aid of two rollersrotating at different rates corresponding to the desired stretchingratio. For the transverse stretching, use is generally made of anappropriate tenter frame.

[0049] The temperature at which the orientation procedure is carried outcan vary over a relatively wide range and depends on the propertiesdesired in the film. In general, the longitudinal stretching is carriedout at from 80 to 130° C., and the transverse stretching at from 90 to150° C. The longitudinal stretching ratio is generally in the range from2.5:1 to 6:1, preferably from 3:1 to 5.5:1. The transverse stretchingratio is generally in the range from 3.0:1 to 5.0:1, preferably from3.5:1 to 4.5:1.

[0050] During the subsequent heat-setting, the film is held for from 0.1to 10 s at a temperature of from 150 to 250° C. The film is then woundup in a conventional manner.

[0051] A great advantage of this process is that it is possible to feedthe extruder with granules, which do not block the machine.

[0052] A further advantage of the invention is that the production costsof the film of the invention are only insignificantly greater than thoseof a film made from standard polyester raw materials. The otherproperties of the film of the invention which are relevant to itsprocessing and its use remain essentially unchanged or have even beenimproved. It has also been ensured that cut material arising directly inthe plant during film production can be used again in the form ofregrind for film production in amounts of up to 60% by weight,preferably from 10 to 50% by weight, based in each case on the totalweight of the film, without any significant resultant adverse effect onthe physical properties of the film.

[0053] The film of the invention has excellent suitability for packagingfood or other consumable items. The film of the invention has excellentbarrier properties, in particular with respect to oxygen. It has beenassured that the individual layers of the laminate remain adhering toone another when the film is processed, e.g. to give film laminates, anddo not delaminate.

[0054] It is also possible to improve the gloss and the haze of thefilm, compared with prior art films. It has been ensured that regrindcan be reintroduced to the extrusion process during production of thefilm in an amount of up to 60% by weight, based on the total weight ofthe film, without any significant resultant adverse effect on thephysical properties of the film.

[0055] The excellent handling properties of the film and its very goodprocessing properties make it particularly suitable for processing onhigh-speed machinery.

[0056] The table below (Table 1) gives the most important filmproperties of the invention again at a glance. TABLE 1 Range accordingto Particularly the invention Preferred preferred Unit Test method Outerlayer A Ethylene 2,6-naphthalate units 90 to 98 91 to 97 92 to 96 % byweight Ethylene terephthalate units <10 <9 <8 % by weight Thickness 0.7μm to 25% of 0.8 μm to 22% 0.9 μm to μm/% by the total thickness of thetotal 20% of the weight thickness total thickness Film properties Oxygenpermeation <85 <80 <75 cm³/(m² · bar · d) DIN 53 380, Part 3 Adhesionbetween the layers >0.5 >0.7 >1.0 N/25 mm internal

[0057] Test Methods

[0058] The following methods were utilized to characterize the rawmaterials and the films:

[0059] Oxygen permeability

[0060] The oxygen barrier test took place using a Mocon Modern Controls(USA) OX-TRAN 2/20, as in DIN 53 380, Part 3.

[0061] SV (standard viscosity)

[0062] The standard viscosity SV (DCA) is measured by analogy with DIN53726, at 25° C. in dichloroacetic acid. The intrinsic viscosity (IV) iscalculated from the standard viscosity as follows

IV=[η]=6.907·10⁻⁴ SV (DCA)+0.063096 [dl/g].

[0063] Coefficient of friction

[0064] The coefficient of friction was determined to DIN 53 375. Thecoefficient of sliding friction was measured 14 days after production.

[0065] Surface tension Surface tension was determined using what isknown as the ink method (DIN 53 364).

[0066] Haze

[0067] The haze of the film was measured to ASTM-D1003-52. The Holz hazemeasurement was determined by analogy with ASTM-D1003-52, but, in orderto utilize the ideal measurement range, measurements were taken on fourpieces of film laid one on top of the other, and a 1° slit diaphragm wasused instead of a 40 pinhole.

[0068] Gloss

[0069] Gloss was determined to DIN 67 530. The reflectance was measured,this being a characteristic optical value for a film surface. Based onthe standards ASTM-D523-78 and ISO 2813, the angle of incidence was setat 20° or 60°. A beam of light at the set angle of incidence hits theflat test surface and is reflected and/or scattered thereby. Aproportional electrical variable is displayed representing light rayshitting the photoelectric detector. The value measured is dimensionlessand must be stated together with the angle of incidence.

[0070] Glass transition temperatures T_(g)

[0071] The glass transition temperatures T_(g)1 and T_(g)2 weredetermined with the aid of DSC (differential scanning calorimetry) onfilm specimens. A DuPont DSC 1090 was used. The heating rate was 20K/min, and the specimen weight was about 12 mg. The glass transitionT_(g)1 was determined in the first heating procedure. Many of thespecimens showed an enthalpy relaxation (a peak) at the beginning of thestep-like glass transition. The temperature taken as T_(g)1 was that atwhich the step-like change in heat capacity—ignoring the enthalpyrelaxation peak—achieved half of its height in the first heatingprocedure. In all cases, there was only a single glass transition stagein the thermogram in the first heating procedure. It is possible thatthe enthalpy relaxation peaks obscured the fine structure of thetransition, or that the resolution of the device was not adequate toseparate the small, “blurred” transitions of oriented, crystallinespecimens. In order to eliminate their heat history the specimens wereheld at 300° C. for 5 minutes after the heating procedure, and thenquenched with liquid nitrogen. The temperature for the glass transitionT_(g)2 was taken as the temperature at which the transition reached halfof its height in the thermogram for the second heating procedure.

[0072] Adhesion between the layers

[0073] Prior to adhesive bonding, the specimen of film (300 mm long×180mm wide) of the present invention is placed on a smooth piece of card(200 mm long×180 mm wide; about 400 g/m², bleached, outer laps coated).The overlapping margins of the film are folded back onto the reverseside and secured with adhesive tape.

[0074] For adhesive bonding of the film according to the presentinvention, use is made of a standard polyester film of 12 μm thickness(e.g. Melinex 800), and a doctor device and doctor bar No. 3 fromErichsen, applying about 1.5 ml of adhesive (Novacote NC 275+CA 12;mixing ratio: 4/1+7 parts of ethyl acetate) to the outer layer A of thefilm of the present invention. After aerating to remove the solvent, thestandard polyester film is laminated to outer layer A of the film of thepresent invention using a metal roller (width 200 mm, diameter 90 mm,weight 10 kg, to DIN EN 20 535). The lamination parameters are: Amountof adhesive: 5 +/−1 g/m² Aeration after application of adhesive: 4 mm+/−15 s Doctor thickness (Erichsen): 3 Doctor speed level: about 133mm/s Bond curing time: 2 h at 70° C. in a circulating air drying cabinet

[0075] A 25±1 mm strip cutter is used to take specimens about 100 mm inlength. About 50 mm of composite is needed here, and 50 mm of unbondedseparate laps for securing/clamping the test specimen. The testspecimens are secured to a sheet metal support by means of double-sidedadhesive tape, by way of the entire surface of the reverse side of thefilm of the present invention (base layer B or outer layer C). The sheetwith the composite adhesively bonded thereto is clamped into the lowerclamping jaw of the tensile test machine. The clamp separation is 100mm. The unlaminated end of the standard polyester film is clamped intothe upper clamping jaw of the tensile test machine (e.g. Instron, Zwick)so that the resultant peel angle is 180°. The average peel force in N/25mm is given, rounded to one decimal place. Specimen width: 25 mmPretensioning force: 0.1 N Test length: 25 mm Separation rate untilpretensioning force applied: 25 mm/min Start position: 5 mm Testdisplacement: 40 mm Sensitivity: 0.01 N Separation rate: 100 mm/min

[0076] The peel force test result is equivalent to the minimum adhesionbetween the layers, since the adhesion between the adhesive and thestandard film is markedly greater. A UV lamp, for example, can be usedto demonstrate the release of the outer layer A from the base layer B ofthe film of the present invention. The UV light has a blueish appearanceif copolymer of PEN and PET is present on the adhesive and this layer isirradiated using a UV lamp.

EXAMPLES

[0077] The following examples illustrate the invention. Information oneach of the products used (trademark and manufacturer) is given onlyonce, and this is then applicable to the examples which follow.

Example 1

[0078] Chips of polyethylene terephthalate and polyethylene2,6-naphthalate in a mixing ratio of 3:97 were dried at a temperature of160° C. to residual moisture below 100 ppm, and fed directly to theextruder for the outer layer A. There, the two materials were extrudedat around 300° C. The melt was filtered, shaped to a flat film in acoextrusion die and placed as outer layer A over the base layer B. Thecoextruded film was ejected via the die lip and solidified on a chillroll. The residence time of the two polymers in the extrusion was about5 minutes. A copolymer was formed in the extrusion under the statedconditions.

[0079] Chips of polyethylene terephthalate were dried at a temperatureof 160° C. to residual moisture below 100 ppm and fed to the extruderfor the base layer B. In addition, chips of polyethylene terephthalateand antiblocking agent were likewise dried at 160° C. to residualmoisture of 100 ppm and fed to the extruder for the outer layer C. Theconditions in the extruder for the outer layer C were the same as thosefor the coextruder A.

[0080] Coextrusion followed by stepwise longitudinal and transverseorientation was used to produce a transparent three-layer ABC film witha total thickness of 12 μm. The thickness of the outer layer A was 1.1μm and that of the outer layer C was 1.0 μm.

[0081] Outer Layer A: 97% by weight of polyethylene 2,6-naphthalate ( ®Polyclear P 100 prepolymer from KOSA, Offenbach) with an SV of 600, and3% by weight of polyethylene terephthalate with SV of 800. Base layer B:70% by weight of polyethylene 2,6-naphthalate ( ® Polyclear P 100prepolymer from KOSA, Offenbach) with an SV of 600, and 100% by weightof polyethylene terephthalate (4020 from KOSA, Offenbach) with SV of800. Outer layer C: 80% by weight of polyethylene terephthalate with SVof 800, and 20% by weight of masterbatch made from 99.0% by weight ofpolyethylene terephthalate and 1.0% by weight of silica particles, 50%of which had an average particle size of 2.5 μm and 50% of which had anaverage particle size of 0.4 μm.

[0082] The individual steps of the process were: Extrusion Temperatures:Layer A: 300° C. Layer B: 300° C. Layer C: 300° C. Take-off rolltemperature  30° C. Longitudinal stretching temperature: 122° C.Longitudinal stretching ratio: 4.5:1 Transverse stretching temperature:125° C. Transverse stretching ratio: 4.0:1 Setting temperature: 230° C.

[0083] The film had the oxygen barrier required and the adhesionrequired between the layers.

Example 2

[0084] Coextrusion was used as in Example 1 to produce a three-layer ABCfilm with a total thickness of 12 μm. The thickness of the outer layer Awas 1.3 μm, and the thickness of the outer layer C was 1.0 μm. Outerlayer A: 95% by weight of polyethylene 2,6-naphthalate ( ® Polyclear P100 prepolymer from KOSA, Offenbach) with an SV of 600, and 5% by weightof polyethylene terephthalate with SV of 800. Base layer B: 100% byweight of polyethylene terephthalate (4020 from KOSA, Offenbach) with SVof 800. Outer layer C: 80% by weight of polyethylene terephthalate withan SV of 800, and 20% by weight of masterbatch made from 99.0% by weightof polyethylene terephthalate and 1.0% by weight of silica particles,50% of which had an average particle size of 2.5 μm, and 50% of whichhad an average particle size of 1.0 μm.

[0085] The process conditions for all of the layers were as in Example1.

Comparative Example 1c

[0086] A film was produced by analogy with Example 8 of EP-A-0 878 297.The film had the oxygen barrier required, but the adhesion betweenlayers A and B was extremely low.

Comparative Example 2c

[0087] A film was produced by analogy with Example 1 of U.S. Pat. No.5,795,528, except that unlike in the example from the U.S. Pat. No.5,795,528 there were only 2 layers selected from PEN and PET. The filmhad the oxygen barrier required, but the adhesion between layers A and Bwas extremely low.

[0088] Table 3 gives the properties of the films produced in Examples 1and 2 and in the Comparative Examples 1c and 2c. TABLE 2 Examples 1 2 1c2c Ethylene 2,6-naphthalate 97 95 100 100 units in the outer layer (in %by weight) Ethylene terephthalate units 3 5 0 0 in the outer layer A (in% by weight)

[0089] TABLE 3 Layer Gloss Film thicknesses Oxygen Adhesion (20° C.measure- Example thickness A/B/C Film permeation between layers mentangle) No. (μm) (μm) structure (cm³/m²bar d) N/25 mm Side A Side C Haze1 12 1.1/9.9/1.0 ABC 78 0.6 205 175 1.8 2 12 1.3/9.7/1.0 ABC 79 1.4 199180 1.6 1c 12 3.0/7.5/1.5 ABC 50 0.1 203 175 1.8 2c 12 6.0/6.0 AB 45 0.1200 195 2.0

What is claimed is:
 1. A transparent, biaxially oriented polyester filmwith a base layer B, at least 80% by weight of which is composed of athermoplastic polyester, and with at least one outer layer A, whereinthe outer layer A is composed of a copolymer or of a mixture ofhomopolymers and copolymers, which contains ethylene 2,6-naphthalateunits in a range of from 90 to 98% by weight and up to 10% by weight ofethylene terephthalate units, and/or units derived from cycloaliphaticor aromatic diols and/or dicarboxylic acids; the thickness of the outerlayer A is more than 0.7 μm and makes up less than 25% by weight of thetotal film, and the T_(g)2 value of the polyester film is above theT_(g)2 value of the polyester for the base layer B but below the T_(g)2value of the polyester for the outer layer A.
 2. The transparent film asclaimed in claim 1, wherein the copolymer or the mixture of homopolymersand copolymers in the outer layer A contains ethylene 2,6-naphthalateunits in a range of from 91 to 97% by weight.
 3. The transparent film asclaimed in claim 1, wherein the outer layer A has a thickness of morethan 0.8 μm and makes up less than 22% by weight of the total film. 4.The transparent film as claimed in claim 1, wherein the oxygenpermeation of the film is below 85 cm³/(m²•bar•d).
 5. The transparentfilm as claimed in claim 1, wherein the adhesion between the individuallayers is greater than 0.5 N/25 mm.
 6. The transparent film as claimedin claim 1, which additionally comprises an intermediate layer Z havinga thickness above 0.1 μm.
 7. The transparent film as claimed in claim 1,the structure of which has three layers and comprises a base layer B, anouter layer A and an outer layer C.
 8. The transparent film as claimedin claim 1, the structure of which has four layers and comprises anouter layer C, arranged thereupon a base layer B, and arranged thereuponan intermediate layer z, and arranged thereupon an outer layer A.
 9. Thetransparent film as claimed in claim 1, wherein at least one of theouter layers has been pigmented.
 10. The transparent film as claimed inclaim 1, wherein at least one side of the film has been treated with anelectric corona discharge.
 11. The transparent film as claimed in claim1, wherein at least one side of the film has been in-line coated. 12.The transparent film as claimed in claim 1, which, at least on the outerlayer A, has been metallized or ceramic-coated.
 13. A process forproducing the film as claimed in claim 1, encompassing the stepsproducing a film from base and outer layer(s) by coextrusion, biaxiallystretching the film, and heat-setting the stretched film, whichcomprises carrying out the biaxial stretching by a longitudinalstretching of the film at a temperature in the range from 80 to 130° C.and by a transverse stretching in the range from 90 to 150° C. and usinga longitudinal stretching ratio in the range from 2.5:1 to 6:1 and usinga transverse stretching ratio in the range from 3.0:1 to 5.0:1.
 14. Theprocess as claimed in claim 13, wherein, for heat-setting, the stretchedfilm is held for a period of from about 0.1 to 10 s at a temperature offrom 150 to 250° C.
 15. The process as claimed in claim 13, wherein cutmaterial arising during film production is reused as regrind in the filmproduction in amounts of up to 60% by weight based in each case on thetotal weight of the film.